The separation of oil from emulsions in water is of great utility in oil production, in oil shipping where ballast tanks and bilges generate emulsions and in industry where oil emulsions are used for cutting and cooling.
If the residual water is to be reused or disposed of in sewers or waterways, the degree of oil removal must be such that the residual water contains less than 10 parts per million of hydrocarbon oil.
It is also desirable that the separated oil be free of water since water often contains salts and causes corrosion. Although oil can contain 40% water and still burn, the chimney steam loss reduces the calorific value and the salts fuse on boiler tubes, leading to tube failure and fusion of furnace walls.
The "Oil/Water Separation State-of-the-Art" publication prepared for Industrial Environment Research Laboratories in Cincinnati, Ohio by Rutgers State University, New Brunswick, N.J., U.S. Department of Commerce National Technical Information Service P.B--280755 is a thorough review of the problem and of updated separation procedures.
Few known procedures are successful when the oil emulsion is stabilised by a surface active agent (surfactant), especially when the maximum allowable oil content of the separated water is 10 parts per million. Complex chemical, physical and biological methods, often all three in sequence, are needed if the water must also meet rigid environmental specifications for detergents.
Oil/water/surfactant blends occur on a huge scale in ship bilges and ballast tanks. A surfactant can enter the ship system from deck cleaners, oil dispersers, laundry wastes, fire foams and deliberate addition to aid cleaning of massive oil storage tanks. Moreover, a surfactant is an essential ingredient of industrial cutting and cooling emulsions and associated rinsing liquors.
In all these uses, the surfactant concentration and even the chemical nature of the surfactant are very variable due to sporadic need or uncontrolled dilution with fresh or salt water. Ship requirements indicate the need for an on-board system so that water, free of oil, but still containing biodegradable surfactants can be released at sea, rather than be brought to shore where dockside waters cannot accept the detergents and other soluble contaminants which may arise from chemical and biological attack on the oil in the bilges. For example, poisonous hydrogen sulphide may be formed and, if so, needs immediate removal along with other biological, soluble products while still at sea.
Recently, ultra-filtration has been used with some success for these surfactant stabilised emulsions. In principle, the oil is retained by its inability to flow through the very fine hydrophilic pores of the ultra-filter membrane whilst water passes under quite low pressure. The oil retention is by a combination of geometry and surface tension. The oil breakthrough pressure, P, is given by: ##EQU1## Where: S is the oil/aqueous interfacial tension,
a is the contact angle of the continuous phase of the pore fluid with the pore wall, PA1 d is the pore diameter. PA1 (i) adding a pre-selected hydrophilic colloid to the emulsion, and, PA1 (ii) passing the emulsion and colloid through a cross-flow ultra-filter containing a polymeric porous membrane.
Surfactants lower the oil/aqueous interfacial tension S and cause breakthrough of oil at even low pressures. The interaction is complex since the surfactant forms micellar structures with itself and with the oil. The critical micellar concentration depends on surfactant composition, on pH, on salts and on temperature.
The net effect of all this complexity is that ultra-filters are usually designed with some latitude to handle these factors by making the pores of the membrane as small as can be to pass some acceptable low flow of permeate water output. This low flow rate must then apply (even though the input emulsion could accept a larger pore size and hence a larger output) because the pore size is relatively fixed--this is wasteful and uneconomic. One solution to this problem is to have a plurality of filter cartridges of different pore size and to use surface tension to indicate when it is safe to bring into use the larger pores. However, the larger pores must remain totally unused as soon as the surface tension drops.
A major problem with all oil/water ultrafilters arises because they are used in the "cross-flow" mode--that is, the feed flows across the ultrafilter where some water permeates, but most of the emulsion (now richer in oil) returns to the feed. Thus, the oil concentration continuously rises which always reduces permeation rate, but this is not the worst effect. Most ultrafilters also show some rejection of surfactant so that the surfactant concentration also rises rapidly in the diminishing recycle aqueous phase. Hence all the water cannot be substantially removed before oil breaks through. Even ultrafilters with pores rejecting over 99% of ovalbumin of molecular weight 45,000 cannot bring the oil concentration above 50% in the presence of most surfactants.
It is an object of this invention to overcome the aforementioned problems which arise in the ultra-filtration of oil emulsions by providing a membrane for the filter in which the pore size is relatively coarse to give relatively larger flow rates when the emulsion is free or substantially free of surfactant or other additive and in which the pore size will reduce upon fall in surface tension when the surfactant or other additive is present.